How to determine source ip address

[Padraig Brady <padraig@xxxxxxxxxxxxx>]

Hi,

I'm writing a network app (currently on linux 2.2.18,
but soon moving to 2.4) that needs to build up ip &
tcp packets manually. So my question is how do I determine
the source ip address to use for a given destination
ip address. I.E. take account of multiple interfaces/routes/
aliases/...

Could I create a temporary UDP socket and connect() it.
If indeed the kernel fills in the appropriate info when
"connecting" a UDP socket (source ip/port), how do I read
this info?

If a port number is not allocated with the above connect(),
how do I manually allocate a source port, bind()?

Or do I have to resort to ioctl()?

thanks,
Padraig.

(I'm not on netdev).